text: The isopoly-molybdenum blues have been known for many years. They are the cause of the "blue waters" found near Idaho Springs, known to Native Americans. They were first documented by Scheele and Berzelius. The compounds responsible for the blue colour were not known until 1995. Before then it was well known that there were polymolybdates of Mo(VI). Molybdenum(VI)oxide, MoO, when dissolved in aqueous alkali forms the tetrahedral molybdate anion, . Dissolving molybdate salts in strong acid produces "molybdic acid", MoO·2HO. In between these extremes of pH, polymeric ions are produced which are mostly built from MoO octahedral units sharing corners and edges. Examples include ,  and , which contain the {(Mo)Mo}-type unit comprising a central MoO pentagonal bipyramid sharing edges with five MoO octahedra. The later unit occurs also in the giant mixed-valence molybdenum blue species [HMoO(HO)(SO)] (x ≈ 16)  as well as in the cluster described in the next section. The molybdenum blue species are obtained by reduction of acidified molybdate(VI) solutions.